In multiple input multiple output (MIMO) communications systems, multiple antennas are used on both the base station and on the mobile device to exploit a phenomenon known as multipath propagation in order to achieve higher data rates. In general, MIMO communications systems simultaneously send and receive multiple data signals over each radio channel. The multipath propagation phenomenon is the result of environmental factors that influence the data signals as they travel between the base station and the mobile device, including, for example, ionospheric reflection and refraction, atmospheric ducting, reflection from terrestrial objects and reflection from bodies of water. Because of these factors, the data signals experience multipath interference that results in constructive interference, destructive interference, or fading, and phase shifting of the data signals. MIMO technology has been standardized in various wireless communications standards including Institute of Electrical and Electronics Engineers (IEEE) 802.11n, IEEE 802.11ac, HSPA+ (3G), WiMAX (4G) and Long Term Evolution (LTE) standards.
MIMO communications systems require testing. A typical MIMO test system for performing “conducted” testing of a base station includes a user equipment (UE) device or UE device emulator, the base station device under test (DUT), a test system computer, and various electrical cables for interconnecting the components. The antenna ports of the UE device or device emulator are typically connected to input ports of the fading emulator by electrical RF cables, although they in some cases they are electromagnetically coupled to the input ports of the fading emulator via a radiated air interface. Output ports of the fading emulator are connected to the DUT. The testing is referred to as “conducted” testing due to the wired connection between the output ports of the fading emulator and the DUT. The test system computer is typically connected to UE device or UE device emulator and to the fading emulator by respective electrical data cables, e.g., Ethernet cables. The test system computer is is in communication with the base station DUT. During OTA testing, the test system computer receives information from the base station DUT that the test system computer processes to evaluate the transmit and/or receive capabilities of the base station DUT.
The next generation of wireless infrastructure (e.g., base stations, backbone, etc.) and customer handsets is called 5th generation mobile networks or 5th generation wireless systems, referred to hereinafter as “5G”. 5G is very ambitious standard that involves millimeter-wave frequency usage, compact phased arrays, and an unprecedented amount of electronic integration. Not only will the transmitters and receivers be integrated into transceivers, but transceivers will be integrated with patch antennas or antenna arrays. This will be the case for both the UE devices and for the base stations. The integrated transceiver and antenna or antenna array is referred to hereinafter as an “integrated transceiver-antenna assembly.”
In the 5G integrated transceiver-antenna assembly of the base stations and UE devices, there will be no traditional connector from the radio electronics to the antenna elements. Because the antenna elements will be very small and there will be a very large number of them integrated together with other electrical components on the same circuit board, external connectors for interfacing the test system with the antenna elements will not be available. For example, the transceiver-antenna assembly may be integrated in the same printed circuit board (PCB) package or ball grid array (BGA) package. In other words, the entire radio, including its antenna or antenna array and its transceiver, will be a single indivisible unit. For these reasons, 5G base stations and UE devices will not be able to be tested by typical MIMO test systems that perform conducted testing.
Nevertheless, radio manufacturers will want their units tested for all of the usual characteristics, e.g., receiver sensitivity, both without and with interference present, total transmit power, error vector magnitude (EVM) of modulation formats, antenna radiation pattern, etc. All of these parameters must be measured and studied in great detail during the product design phase. The non-separable nature of an integrated transceiver-antenna assembly renders traditional transceiver testing methods useless.
The best MIMO test system that is currently available for testing base station DUTs is a multi-probe anechoic chamber (MPAC) over-the-air (OTA) test system. In a typical MPAC OTA system, the base station DUT is located inside of a large anechoic chamber that also has a multi-probe antenna element configuration. The antenna elements of the base station DUT are not required to be physically connected to the output ports of the fading emulator. Rather, the probe antenna elements of the multi-probe antenna element configuration are connected to the output ports of the fading emulator to allow OTA testing rather than conducted testing of the base station DUT to be performed. However, the MPAC OTA test system has drawbacks in terms of cost and space requirements. One drawback is that the MPAC OTA testing method is a radiating far-field testing method that requires that probe antennas be positioned in the radiating far-field zone of the base station DUT, which, in the case of massive MIMO test systems and high frequencies (e.g., 28 GHz), may be several meters. Consequently, the anechoic chamber must be relatively large, typically requiring at least ten square meters of floor space, which leads to the chamber being very expensive.
The MPAC OTA test system also requires many probe antennas and many fading emulator channels to feed the probe antennas. The number of required probe antennas increases as a function of the number of clusters that are in the channel model, and in a multi-user case, also as a function of the number of users. Furthermore, a dynamic channel model that employs dynamic cluster angle evolution over time requires a very high number of probe antenna elements even in a single-user case that uses a relatively simple channel model. Consequently, it is anticipated that a MPAC OTA test system for testing 5G base stations will be extremely expensive due to the requirements for a very large anechoic chamber and an emulator having a very large number of channels.
A need exists for a robust OTA MIMO test system and method that can be used to test base stations and UE devices that do not have connectors for connecting the test system to the antenna ports of the base station or UE device and that can be achieved at relatively low cost. A need also exists for such a MIMO test system and method that eliminate the need for a large anechoic chamber, that eliminate the need for the fading emulator to have a very large number of channels, and that eliminate the need for the probe antenna element array configuration to have a very large number of probe antenna elements.